This invention relates in general to vehicular hydraulic brake systems and in particular is concerned with methods to improve braking performance in such systems during low temperature conditions.
Electronically-controlled hydraulic brake systems for vehicles are well known. A typical system includes a master cylinder, fluid conduit arranged into a desired circuit, and wheel brakes. The master cylinder generates hydraulic forces in the circuit by pressurizing brake fluid when the driver steps on the brake pedal. The pressurized fluid travels through the fluid conduit in the circuit to actuate brake cylinders at the wheel brakes and slow the vehicle.
Electronically-controlled hydraulic brake systems also include a hydraulic control unit (HCU) containing control valves and other components located between the master cylinder and the wheel brakes. Through an electronic controller, the control valves, a pump, and other components selectively control pressure to the wheel brakes to provide a desired braking response of the vehicle, including anti-lock braking, traction control, and vehicle stability control.
During anti-lock brake events, a driver is applying a brake pedal and thus pressurizing fluid via a master cylinder. This pressurized fluid is available for re-apply events that selectively permit pressurized fluid to reach the wheel brakes. A pump in the HCU draws fluid from the wheel brakes during a dump cycle and directs fluid to the wheel brakes during a re-apply cycle. Thus, pressurized fluid is available from both the master cylinder and the pump during a re-apply event.
During traction control and vehicle stability control events, a driver is usually not applying a brake pedal and thus the master cylinder does not provide pressurized fluid to the wheel brakes. Instead, the pump in the HCU is activated and provides a sole source of pressurized fluid available to the wheel brakes. A pump inlet can be placed in fluid communication with a fluid reservoir by selectively switching control valves mounted in the HCU.
Performance of the brake system can be adversely affected by various factors, including flow resistance at the inlet side of the pump. Primary sources of resistance on this suction side of the pump include the master cylinder, brake lines from the master cylinder to the HCU and from the HCU to the wheel brakes, and the HCU itself. In particular, the control valves and other components, along with the various fluid passages formed in the HCU, create a significant restriction.
Low temperature can also adversely affect the performance of the brake system. The viscosity of the brake fluid increases as the temperature decreases. High viscosity of the brake fluid at low temperature, particularly at temperatures belowxe2x88x9220 C, impacts the ability of a pump to draw fluid. Low temperatures, combined with the above-discussed restrictions, can result in undesirable pump performance.
This invention includes an electronically-controlled vehicular hydraulic brake system that provides anti-lock braking, traction control, and vehicle stability control. The system includes a hydraulic control unit (HCU) containing control valves and other components in fluid communication with wheel brakes. Low temperatures cause the viscosity of hydraulic fluid to increase. Performance of the braking system can be improved at low temperatures according to methods of this invention.
A first preferred method of improving braking performance is to increase the brake fluid temperature inside the HCU. Several examples of methods to increase fluid temperature inside the HCU are presented.
A second preferred method of improving braking performance is to preload the wheel brakes with a pressure that does not adversely affect normal driving. Several examples of methods to preload the wheel brakes are presented.
Various objects and advantages of this invention will become apparent to those skilled in the art from the following detailed description of the preferred embodiments, when read in light of the accompanying drawings.